1. Field of the Invention
The present invention relates to magnetic recording media to be used in magnetic disk units or the like, and particularly, relates to a magnetic recording medium having a magnetic recording layer formed of a thin ferromagnetic metal or alloy film. More particularly, the present invention relates to high-performance magnetic recording media which exhibit low noise, which are superior in S/N (signal-to-noise ratio) characteristics, suitable for high-density recording and which exhibit excellent recording/reproducing characteristics.
2. Description of the Prior Art
Heretofore, magnetic recording media using metal or metal oxide magnetic thin films have been proposed as high-density magnetic recording media, as described in Japanese Patent Publication No. 54-33523 (1979). As methods of film formation for those magnetic recording media, generally, sputtering, evaporating, ion-plating and the like have been used. Recently, a demand for high recording density has intensified, and practical use of continuous recording media using metal magnetic films have been started with the advance of medium- or small-scaled disk apparatus.
However, those medium- or small-scaled disk units have a disadvantage in that the disk is small in diameter and therefore low in peripheral speed. Accordingly, two attempts to make the output signal high and to make noise low have been required to thereby make the SN ratio larger.
In the conventional magnetic recording media of the ferromagnetic thin film type, noise produced at the time of magnetic recording/reproducing is caused mainly by the occurrence of a region in which the magnetization direction becomes disordered (magnetization transition region) in the boundary of recording bits, as discussed at page 16 in Digests of the Fifth Annual Conference of Magnetics in Japan (Oct., 1981), the Magnetics Society of Japan.
Magnetic recording media of the ferromagnetic thin film type in which ferromagnetic metal thin films or metal oxide thin films are used as magnetic recording layers (simply referred to as magnetic layers) have an advantage in that the coercive force is large, the saturation flux density is high, and the magnetic layer can easily be made. Those properties are essential for performing high-density recording on magnetic recording media. Therefore, those magnetic recording media are expected as next-generation high-density recording media in place of the magnetic recording media of the particulate type which have been used widely. However, the magnetic recording media of the ferromagnetic thin film type have a disadvantage in that the production of media superior in S/N characteristics is difficult because noise in the magnetic recording media themselves is high, though the reproductive output signal can easily be improved.
Several researches have been carried out about the cause of occurrence of noise in the magnetic recording media of the ferromagnetic thin film type. For example, according to the aforementioned literature (Digests of the Fifth Annual Conference of Magnetics in Japan, Oct., 1981, page 16), a magnetization transition region produced in the boundary of recording bits is pointed out as one of the main causes of noise which occurs at the time of signal recording. This suggests that noise increases substantially proportionally to recording density. Accordingly, reduction of the proportional constant is one of the important factors for producing high-S/N magnetic recording media. However, the relation between noise innate in the magnetic recording media and the magnetic characteristics of the magnetic thin film in the media remains unexplained till now. The fact that it remains unexplained has placed a hurdle on the road to production of high-S/N magnetic recording media.
On the other hand, heretofore, with respect to metallic thin film media, the surface form of the media has been smoothed by reducing sputtering gas pressure for the purpose of improvement in corrosion resistance, as described in Extended Abstracts, 28a-ZH-3 (The 47th Autumn Meeting), The Japan Society of Applied Physics. However, according to the research by the inventors of this application, noise innate in the magnetic recording media having the smooth surfaces is still high so that the recording/reproducing characteristics thereof are not sufficient. It is therefore important to improve those characteristics. Further, so-called "modulation" (modulation of read-signal envelope), exists in the metallic thin film media, as described in Journal of Vacuum Science Technology, A4 (1986), pages 547 to 549. Accordingly, those metallic thin film media are not sufficient in uniformity and reliability.